In response to my Bible study on Hebrews 12 today:
When we understand that God is using our circumstances to discipline us for our good (verse 10) — to share in His holiness (v. 10), that we may bear the peaceful fruit of righteousness (v. 11), and that we may live and see God by His holiness in us (v. 14) — then there is joy in our circumstances! We can see God demonstrating His love for us as His sons (vv. 5-8) and we find there is no reason for bitterness to spring up (v. 15).
But sometimes, we just like to indulge in the bitter!